1. Field of the Invention
The present invention relates to a GaN based compound semiconductor light-emitting device (LED) and a manufacturing method therefor, and particularly to a GaN based compound semiconductor light-emitting device (LED) with better light transparency and a manufacturing method therefor.
2. Description og Related Art
A light-emitting device (LFES) has been generally known as a device with ability to light generating, which has been widely used in digital watches, calculators, communications and other areas, such as mobile phone and some appliances. Recently, the efforts and attempts have shifted to use LEDs in more ordinary human living, such as large panels, traffic lights and lighting facilities. However, in marching into a brand new era replacing the current lighting facilities with LEDs, the luminous efficiency of an LED is still a significant issue, which has been challenging those skilled in the art for many years. Therefore, many developments and researches have been thrown in to improvement of luminous efficiency of LEDs, and red, green, blue and white colored lights are alike.
As is well understood to those skilled in the art, LEDs are produced based on some semiconductor materials, especially GaN-based compound semiconductor, and emits lights by virtue of the behaviors featured in the semiconductor materials in the presence of an applied electrical bias.
In particular, an LED is generally composed of some III–V group (or II–VI group, although rarely given forth) compound semiconductors accounting for their stronger inclination of recombination of electrons and holes. In principle, an LED is basically a well-known p-n junction structured device, i.e., a device having a p region, an n region and a depletion region therebetween. With a forward voltage or current bias applied, the majority of the carriers in the p or n regions drift respectively towards the other region through the depletion region in the device due to the energy equilibrium principle and a current is accounted for, in addition to the general thermal effects. When some electrons and holes in the device jumped into a higher value of energy band with the aid of electrical and thermal energy, the electrons and the holes recombine there and then give off lights when they randomly fall back to a lower energy state (turning from al unsteady state to a steady state) owing to thermal equilibrium principle, i.e. spontaneous emission. Besides the p-n junction, in a typical and basic such device stricture there are also other components, such as a substrate, a buffer layer, a transparent contact layer (TCL) and electrodes. In achieving a high luminous efficiency LED, each component and their mutual relationship in the device structure are generally considered.
In a typical LED, a TCL is a layer coated on the LED structure and below a p-type electrode. Since the p-type electrode is normally not transparent and will have blockage on the emitted light to a user's eyes, the p-type electrode should be sized and disposed at a limited portion on the underlying layer contact therewith. However, the electrical force lines resulted from between the p-type electrode and an n-type electrode may not uniformly distribute in the p-n structure in the device. Hence, the electrical charges provided by the applied electrical bias may not efficiently stimulate the p-n structure, which is necessary for light generation. Further, the p-type electrode is inhered with poor mobility as compared to that of the n-type electrode and thus the stimulation efficiency of the electric bias on the device may not be satisfactory. A thin TCL is in this occasion coated over the toppest layer (in fact, under the p-type electrode). The TCL is a transparent material to a light generated from the device and equipped with ability of electricity conduction. Once an electric bias is fed from the p-type electrode, the corresponding charges will spread uniformly in the p-n structure with an aid of the TCL underlying the p-type electrode and the poor stimulation efficiency of the electric bias may be overcome.
Ni/Au material is widely used as the TCL in a GaN based light-emitting device in achieving an improved light-emitting device. However, Ni/Au is not a material with satisfactory light transparency and should thus be made considerably thin, about 0.005–0.2 μm. However, according to the critical angle theory, a TCL should possess a suitable thickness and will then facilitate extraction of the generated light out of the device. Therefore, Ni/Au material may not be the most appropriate choice as a TCL for an LED in light transparency and extraction efficiency's view owing to the thickness issue. Further, since such GaN based light emitting device with Ni/Au as the TCL may not be formed with more facets by use of surface treatment under the limitation of 0.005–0.2 μm of thickness of the Ni/Au layer, the light extraction efficiency stands little possibility to be promoted in terms of the Ni/Au layer.
In view of the foregoing problems, it is needed to set forth a GaN based compound semiconductor LED that may really provide an improved TCL. To this end, the inventors of the present invention provide herein a GaN based compound semiconductor LED with a TCL other than Ni/Au. To further enhance the function the TCL may provide, a suitable adaptive layer for the TCL is provided in the LED structure whereby the entire device may achieve better light transparency and extraction efficiency. Thus, the combination of the TCL and its adaptive layer may well replace the currently used Ni/Au TCL.